1. Field of the Invention
The subject invention relates to gas turbine engines, and more particularly, to a system for transferring residual fuel from the engine manifold of a gas turbine engine to prevent fuel from coking in the engine manifold when the engine is shut down.
2. Background of the Related Art
In aircraft gas turbine engines, it is desirable to prevent residual fuel from remaining in the engine manifold valve after the engine has shutdown so as to prevent coking of the residual fuel. Coking of the residual fuel presents a fire hazard and can cause blockages in the engine fuel injectors which can shorten injector life and harm engine performance.
It is known in the art to store residual fuel in a drain can or container, with the container being connected to the engine manifold through a valve which opens when the engine is shut down. With the valve open, the system is arranged to drain the residual fuel into the container using gravity and/or the pressure differential between the engine manifold and the container. Since the amount of residual fuel drained into the container at each engine shutdown may be as much as one gallon or more, it is generally necessary, due to the size limitations of the container, to empty the container between engine shutdowns.
While numerous methods or systems for emptying such containers between engine shutdowns have been employed, they generally suffer from the disadvantage of either emptying the contents of the container into the atmosphere upon takeoff or requiring the aircraft ground crew to manually empty the container. For example, one prior art method involves directing ram pressure to the container to force the collected fuel to drain to the atmosphere upon aircraft takeoff. With increased air traffic, high population density in the vicinity of most airports, and greater environmental awareness, such atmospheric dumping systems have become unacceptable.
U.S. Pat. No. 3,774,394 to Criffield discloses another system for draining residual fuel from the manifold of an aircraft engine, downstream of the engine shutoff valve, to prevent the fuel from coking. The system includes an eductor pump and valve assembly, with the valve assembly being operative to direct pressurized fuel to the eductor pump as motive fluid therefor during engine spin-down. A drain valve is provided which opens through the actuation of mechanical or electrical means when the engine shutoff valve is closed, so as to drain the fuel downstream of the shutoff valve into a drain can. The eductor pump is connected to the drain can and the output of the high pressure engine fuel pump through the valve assembly is used to pump residual fuel from the drain can to the fuel supply system of the aircraft. The valve assembly is a relatively complex mechanism that includes a reciprocating piston carrying a spring loaded plunger that acts as a check valve to prevent the reverse flow of fuel from the eductor to the drain can as the engine coasts down and the eductor pump action is lost.
While the Criffield patent provides a unique system for pumping the residual fuel from an engine manifold after the engine is shut down, it is a relatively complex system. In particular, the system requires a check valve to prevent fuel from flowing back into the drain can when the pumping action of the eductor becomes insufficient to maintain the piston in its open position. It also requires the sequential actuation of the engine shutoff valve and the drain valve. These features add to the overall complexity and may decrease the reliability of the system. Accordingly, there is a need for an improved engine manifold drain system which overcomes the deficiencies of prior art systems.
The subject invention is directed to a system for draining residual fuel from the engine manifold of an aircraft gas turbine engine to prevent the fuel from coking. In accordance with a preferred embodiment of the subject invention, the gas turbine is operatively associated with a fuel metering unit having a main fuel pump, for example, a variable displacement vane pump, for delivering high pressure fuel to the engine manifold, a boost pump for delivering low pressure fuel from a fuel tank to the main fuel pump, and a shutoff valve for controlling the flow of high pressure fuel between the engine manifold and the main fuel pump.
The manifold drain system includes an ejector having an inlet port for receiving high pressure fuel from the main fuel pump at engine shut-down, an outlet port and a suction port. The ejector defines a nozzle that extends between the inlet port and the outlet port for increasing the velocity of high pressure fuel flowing therethrough at engine shut-down so as to create suction at the suction port.
The manifold drain system further includes a motive valve and a drain valve. The motive valve controls the flow of high pressure fuel between the main fuel pump and the inlet port of the ejector. It is configured for movement between a first position wherein high pressure fuel is directed from the main fuel pump to the engine manifold during engine start-up and operation, and a second position wherein high pressure fuel is directed from the main fuel pump to the inlet port of the ejector at engine shut-down. The drain valve controls the flow of residual fuel between the engine manifold and the suction port of the ejector. It is configured for movement between a closed position and an open position as the motive valve moves between the first position and the second position. As a result, residual fuel is drawn from the engine manifold under suction to the inlet port of the ejector. In addition, a fuel recycle conduit is provided for delivering the residual fuel from the outlet port of the ejector to the fuel tank.
Preferably, the manifold drain system of the subject invention further includes means for controlling the simultaneous operation of the motive valve and the drain valve. The controlling means includes a three-way solenoid valve that is in fluid communication with the low pressure side of the main fuel pump by a flow restricted fuel control line and is in fluid communication with the high pressure side of the main fuel pump by a siphon line.
In addition, the manifold drain system includes means for facilitating fluid communication between the solenoid valve and the motive valve, drain valve and shutoff valve, such that upon engine shut-down the siphon line is opened to direct high pressure fuel from the main fuel pump to the shutoff valve, motive valve and drain valve, whereby the engine shutoff valve is moved to a closed position, the motive valve is moved to the second position, and the drain valve is moved the open position. Conversely, upon engine start-up the siphon line is closed so that low pressure fuel is directed from the boost pump to the shutoff valve, motive valve and drain valve, whereby the engine shutoff valve is moved to an open position, the motive valve is moved to the first position, and the drain valve is moved to the closed position. Means are also provided for activating the solenoid valve upon engine shut-down so as to open the siphon line.
In accordance with a preferred embodiment of the subject invention, the motive valve and the drain valve define an integral valve assembly having a common valve housing and a common valving member. In certain instances, the integral valve assembly may include a float valve for preventing air and combustion gases from entering the fuel system once the residual fuel has been drained therefrom. However, this mechanism is not required for the efficient operation of the system.
These and other unique features of the manifold drainage system of the subject invention will become more readily apparent from the following description of the drawings taken in conjunction with the detailed description of the preferred embodiments.